There are three main types of light sources widely used today at a domestic/consumer level: incandescents, compact-fluorescents (CFLs), and Light Emitting Diodes (LEDs).
The light efficiency of different lights sources may be measured in terms of their luminous efficacy, which has units of lumens per Watt (lm/W). This is the amount of lumens emitted by a light source for a given wall-plug electrical power fed into the device. A candle has a luminous efficacy of 0.3 lm/W (which is on the low end of the scale) while an ideal monochromatic green laser has a luminous efficacy around 683 lm/W. Such lasers are the most efficient light sources and occupy the high end of the efficacy scale. The amount of luminous power required by a light source to adequately light a small room is around 1000 lumens. The objective of an efficient light source is to reach this amount of luminous power while consuming the smallest possible electrical power.
Incandescent bulbs were the first bulbs ever invented and convert electricity to light. Their main technology relies upon heating a metal filament (usually tungsten) at very high temperatures, until it glows. The technology was first developed over 200 years ago and was commercialized in the late 19th century. It provides a type of light (spectrum) very pleasant to the human eye because it is similar to the spectrum of light emitted by the sun, for which our eyes are mostly accustomed to. This is the spectrum of a black body radiator.
However, incandescent bulbs are very inefficient light sources as their power consumption is extremely high for the light output they produce. For example, 60 W incandescent bulbs have a luminous efficacy around 15 lm/W, thus emitting around 900 lumens of light. The efficacy of incandescent bulbs is so low due to the heating process that takes place. Most of the input electrical power (around 90%) is released as heat at infrared frequencies, rather than as useful visible light.
Compact Fluorescent Lamps were designed to replace incandescent bulbs and operate based on the phenomenon of luminescence, which is the conversion of ultraviolet (UV) light into visible light. The CFLs typically contain inert gases, such as mercury and argon. When electricity is supplied at the end of the tube containing the gases, it forces the mercury atoms in the gas compound to emit energy at UV wavelengths. This UV light is absorbed by the special chemical coating of the tube and is re-emitted via luminescence as visible light. The chemical coating consists of one or more types of phosphors, each responsible for emitting light in a different range of wavelengths.
This method does not involve heating, and thus the luminous efficacy for CFLs is around 70 lm/W, being over 4 times more efficient than incandescents. At the same time, their lifespan is 10 times more compared to incandescents. However, the light quality of CFLs is nowhere near that of the light emitted by incandescents. This is because the spectrum of light emitted by CFLs is totally different compared to the more natural light produced by incandescents. The latest type of light bulbs available is LEDs. These are solid-state devices that emit light when a small direct current (DC) voltage is applied across their terminals. The technology has been available for several decades, and low power LED lights have been routinely used in certain markets such as electronics where bright white light is not always required. In the past decade, LEDs have made a push towards consumer lighting products which are likely to become the most established lighting technology in the near future. This is because the luminous efficacy of LEDs can reach a theoretical maximum of 300 lm/W, which is 20 times higher compared to incandescents and over five times higher compared to CFLs. At the same time, LEDs have a lifespan that can reach 30 years, and so buying a good LED lamp is a safe long-term investment from a consumer's point of view.
Consumer LED lamps available today have been limited to low wattages, typically to “40-Watt equivalent”. This means that such an LED lamp will emit the same light (luminosity) as a 40 W incandescent bulb, even though it will only consume about 5-10 W of electrical power. The typical luminous efficacy of current practical LED lamps is around 70-100 lm/W, comparable to that of CFLs. All LEDs are directional, which is why manufacturers have to add reflectors and various light-diffusing coatings or resin housings to make the light scatter for use in home and other lighting applications.
Each of these types of light sources has its drawbacks. Incandescents have very low efficacy and consume disproportionate large amounts of power compared to their light output. CFLs have improved efficacy, however by sacrificing the light quality, causing discomfort after prolonged exposure. LEDs have excellent efficacy, however their absolute luminous power is lower compared to that of other types of light sources.
The present disclosure relates to addressing some of the current limitations of conventional light sources.